Telephone system



Dec. 25, 1945. K, L BURGENER 2,391,389

TELEPHONE SYSTEM Filed Aug. 4, 1944 2 Sheets-Sheefl 1 OmN INVENTOR.

KARL L. BuRer-:NER

ATTORNEY Dec. 25, 1945. K. L.. BURGENER TELEPHONE SYSTEM Filed Aug. 4,1944 2 Sheets-Sheefl 2 KARL L BURGENER4 ATTORNEY Patented Dec. 25, 1945U NITE D STATES PAT ENT O F lFl CEl 21,391,389 n TELEPHONE SYSTEM KarlL. Burgener. Villa Park,A lllv., assgnor to Automa-tic ElectricLaboratories, Inc., Chicago,r

Ill., a corporation of Delaware Application August 4, 19441,I Serial No.548,077 e claims. ('olg' 17a-2n This inventiony relates tot telephonesystems in general, but is more.V particularly concerned with automatictelephone systems in which both main exchanges and branch exchanges areemployed;

and the broad. general object, briefly stated., may f switches, afterseizure in the mainv exchange by an. automatic. switch or by anVoperator in a manual main exchange. Another method which has been.employed is to bridge across. the trunk a highresistance relay at thebranch exchange. A stilLiu-rther method hasl been to connect a re'- layfrom one trunk. conductor to groundv at the branch exchange. This.inventionv provides in the branch exchange. a relay which is normalyconnected from the ground side. of the trunk to battery 'at the branch..exchange; However during eachcall over the trunk in either direction,the relay is during certain periods, bridged; across the two trunkconductors and is operated by current. lflowing over the trunk from themain exchange, The relay will operate and perform its functionsregardless of the direction of flow of said. current.. n

One. of the objects. of this invention isto provide` improved. means forinstantly busying the trunk circuit at the branch exchange when;- aswitch in the mainy exchange seizes: the trunk circuit orY when anoperator in. a manual exchange seizes the trunk.

Another object of this invention, is to provide means for guarding; thetrunk circuit against intrusion. during the period of time between ac*-tual seizure by the main exchange and the start of the ringing period,which followsl seizure, for operating the answering signal: to attractthe branch operators. attention.

Another object of this invention is to' provide means for guarding thetrunk circuit againstin.- trusion during the period between the` actualcompletion of the conversation: and the return tof normal of all of' theelements in the trunk circuitl A feature of thisin-vention is theprovisionof means for connecting ground to' onef of the trunk conductorsto close a circuit: for operating the ling equipment in the mainexchange to: cause said.v line; equipment. to seize a. trunkv leading:to an. idle: selector at the main. exchange.

Another feature.- of. this invention is the provisiony of means forpreventing the branch exchange subscribers from completing calls to themain exchange numbers in specied. groups by including,r means foropening the trunk circuit, placing ground on one conductor of the mainex.- change section oi the trunk, alternately connect.- ing thehighresistance relay from battery to the otherV conductor and across the twoconductors, to guard the trunk from intrusion until the callinglsubscriber abandons the unauthorized V-call. A distinctive tone may beintroduced onto. `the calling subscribers line: at. this time by theadapter to remind the. cal-ling party that his line is connected to anunauthorized number.

Another feature of this invention isthe provision of a non-ring-th-roughrepeating coil to prelvent ringing current from reaching. the operatorsreceiver in theevent that the answering plug wasv inserted into the jackduring. the. ringing period, khowever after the plug hasonce been placedinthejack. in. answering an incoming call or'in-ithe event that` a.patching cord is placed in the jjack toconnect thetrunk direct to. asubscribersi line.. lthe novelv circuit arrangements permits theringing. current from` the main ex.- changeto by pass the repeatingcoil. to directly vring a supervisory relay in the. cord circuit. or toring directly onto'the; line of the called Vsubscriber.. In ringing in.on the supervisory relay the: main exchange. operator can recallthebranch exchange operator. l

The foregoing objects and features together I with others notspecifically mentioned will be explained in detail hereinafter, withrreference to thev drawingsv comprising Figures.` 1, 2, 3v and 4. The:drawings show' by theA usual circuit diagrams a, suicient amount. of.the equipment in, a.. telephone system embodying the invention to permitthe invention to loereadily explained andun'derstood. Thedrawings shouldbe arranged. horizontally in numerical, order with Fig. 1 atI the left.

The apparatus shown in Fig. 1 is a skeleton key and cord circuitl of aprivate exchange attendants switchboard. toshow the normal condltionI ofa loop across the tip- I-.Bland ring; |62y of thel plug. When. thebranch; exchange operator inserts: plug; Pl into'jack J2 a. loop isplaced across' the branch exchange section of the. trunk. 2L is thetrunk circuit comprising conductors 25T andi '258; They branchV exchangeendY has two branches.. one; terminatesthe. jack J-2 which is usedmostly' for answering mainA to,4 branch exchange. calls, although it maybe used byr the release type.

operator for the purpose of extending special calls from the branch tothe main exchange. This jack may also be used with patching cords toextend the trunk line direct to one of the branch exchange subscriberslines, for night service or special cut-through day service. The mainexchange end also has two branches, one branch to the regular lineequipment for calls incoming to the main exchange from the branchexchange and the other branch to multiple bank contacts for outgoingcalls from main to branch exchange. Fig. 4 is a skeleton of the mainexchange circuits showing the trunk terminating in a line equipment forthe incoming calls and, in multiple switch bank contacts for theoutgoing calls from main to branch exchange.

When the trunk circuit is taken into use at the main exchange of theautomatic type, a connector seizes the trunk and supplies ground andbattery from the ring cut-oi relay to the two sides of the trunk line.Ground, over one side of the trunk from the ring cut-off relay, operatesthe high resistance relay to place a guarding potential on the selectorbank multiple in the branch exchange and to light the busy lamp.Following seizure, ringing current is superimposed on the trunk line tooperate an alternating current relay at the branch exchange to signalthe operator by lighting the answer lamp. When the branch 'exchangeattendant answers the call, the circuit will be closed through a loop inthe cord circuit causing the ring cut-off relay to operate and completethe talking circuit from the calling subscriber to the attendant cabinetoperator.

As the ringing current is applied intermittingly to thek circuit thereis a so called ringing period and a silent period. By providing the highresistance relay connected from battery to the ground side of the trunkline, this relay will operate practically simultaneous with the seizureby vthe connector at the main exchange. Thus the trunk will be guardedfrom the instant it is seized by the main exchange connector regardlessof the condition of the ringing source, as to whether it is in theringing or the silent period. The windfing of this relay is of such acharacter that it can operatefrom ground through one winding of theringcut-oi relay without causing said ring cut- The busy lamp on theatis seized but the answer lamp will not light until `after the ringingcurrent from the connector is superimposed in bridge on the trunk,causing the -alternating current relay to operate and close the circuitto the answering lamp.

After a conversation has been terminated both calling and called partieswill open their line circuits by hanging up their receivers and theswitch train in the central cnice will restore to normal immediately, ifthe connector is of the first party This means that the connector willrestore before the operator could remove the plug from the jack andmeans must be provided to guard the main exchange end of the trunk for aperiod of time until the operator frees the branch j exchange end. Thisis accomplished by the novel circuit arrangement of the trunk wherebythe release of the trunk by the connector in the main exchange, causes arelay in the trunk circuit to operate opening the ground side of thetrunk to the attendants switchboard and placing a ground on the batteryside of the trunk to the main cnice, thus immediately operating a lineequipment in the main office placing a guard on the trunk circuit. Whenthe operator removes the plug from other branch terminating in a regularline switch the jack at the attendants switchboard thus freeing thebranch exchange end of the trunk circuit, the line equipment at the mainexchange will immediately restore making the trunk available for callsin either direction.

Should the branch exchange operator remove the plug before the releaseof the connection in the main oice, the branch exchange end of the trunkmust be guarded from seizure by a subscriber making an outgoing call,This is accomplished by a novel feature of the circuit which causes thehigh resistance relay to be bridged across the trunk circuit and held inoperated position by direct current from the main exchange which currentmay be in either direction. rhis relay in turn causes the busy conditionto be maintained on the multiple bank contacts of the automatic switchesin the branch exchange and also causes the busy lamp to glow to indicatethe busy condition to the branch exchange operator. The trunk circuitwill remain in this condition until current is removed from the mainexchange end of the trunk circuit to allow the high resistance relay tooperate to release other relays and restore the trunk circuit to itsnormal condition and make the circuit available to calls in eitherdirection.

A particular advantage of bridging the high the conditions where callingmain exchangeeither automatic or manualhas not released even though thebranch exchange has released, is that the main exchange end of the trunkmay be attached to a manual switchboard or to any one of several typesof automatic systems or to a Toll switchboard, and the various circuitsinvolved are dissimilar. There are instances where the current iiow fromthe main exchange is in the normal direction of ground on positive lineand battery on negative line when the call is iirst established and at alater stage the current flow is in the exact opposite direction. Thehigh resistance relay, when connected as a bridge at the ,branchexchange end, will be energized in either instance and therefore thistrunk circuit is more universal in its application than the circuitsheretofore used.

Other objects and features of this inventionl are concernedY mainly withcircuit details which will be explained hereinafter with reference tothe 'accompanying drawings, which show by means of Fig. 1 is a part ofthe branch exchange attendants cord cord circuit to show the closed loopfrom the plug tip I6! through one winding |13 of the repeating coil,contact III, relay 120, contact H2, one winding of repeating coil |14 toring 162 of the plug. Fig. 4 is a portion of the main exchange circuits,indicating that the trunk circuit conductors 463 and 464 have two vbranches, one branch terminating in multiple contacts of well knownStrowger switch banks through which the main exchange subscribers haveaccess to the trunk line leading to the branch exchange and theequipment through which the branch exchange subscribers have access tothe main exchange subscribers. It should be noted that while the lineequipment shown is of the well known type, there is a slight change in=the connections, ground has been removed from one spring ot rel'ay/Ilwith the-result that; when this relay is in its normal position,contact, 4 I I' will not connect with ground,

therefore trunk conductor 461i is'free from ground Should Y when the'trunk circuit vis normal. trunk termina-te in a manualk main exchangethe terminating equipment would have circuit characteristics similarl tothose described herein as applying to the automatic system.

Fig. 3 illustrates the discriminating feature, indicating a method ofpreventing the branch exchange` subscriber from completing, by dialing,a connection through thistrunk circuit` to unauthori'zed numbers, andfurther means foropening the trunk circuit and connecting the high"resistance relay from'conductor 463 to battery under certainconditions,` andl under other conditions connecting said' relayacrossthe outgoing section to guard the main exchange end until' the callingbranchA exchange subscriber abandons the call. i

It will be noted thatv the branch exchange end of thev trunk circuitalso has two branches, one branch terminatingin the multiple bankcontacts 251, 258 and 259', through which the branch exe trunks leadingto main exchange., it will be assumed that Fig. 2 illustrates one trunk,in such a group; The other branch terminates in a jack on an operatorsposition ina branche. exchange switchboard or attendants cabinet.``'Ihis branch is` equipped' with the ordinary7 l'amp signals' toindicate incoming calls and busyconditions on` out.- going calls,- andthe usualv jack. into which the operator may insert a plug in answering.incoming calls or for establishing outgoing calls to the main exchange,or for a patching cord by which `a subscribersA line maybe connecteddirect. to a trunkl rornight use or for any other special service. Themain exchange may be manual or auto,- matic andi when the latter, theoperator completes the call by use ofa dial, which is part of an.operators circuit (not shown).

In connection with the drawings it will. be as.- sumeol that asubscriber inthe automatic main exchange desires to converse witha.subscriber in the branch exchange. It will be assumed further that whenthe calling. subscriber in the mai-n exchange removes his receiver .anddials the. correctVA digits of the desired number a well known Strowgerselector with vertical and rotary motion, seizes. the. rst idle trunk.to thefbranch exchange.

Assuming that the. trunk. linefshown isfthe rst idle trunk` accessibleto the selecting. switch, the wipers. of` said switch will then stop. onbank con.- tacts 421, 428 and` 429.` The switching relay 42s!) of theselecting switch willk operate to.. connect the wipers o-f the selectorto the bank contact instantly establishing acircuit from ground through,relay 449, contactsv 44| and 42|, bank contact 421-, trunkv conductor.4.63,. contact 3;I-I, conductor 3.64, contact` k, righ resistance. relay280, contacts 2.94 and 263-, resistance coili 2141 to battery; Relay280.Y operates and closesa. circuit to relay 2m.. from ground.contacts-v 28-.I and 2&9', number -2 wind-ing of rel-ay 2M) tobattery;`Reh lay 2 L0 operates, lighting the busy lar-np' 21:15, from ground,vcontact 2 [-4, lamp 2'I1` to; battery: Relay 2I9 also closes ground'through contact 219 to leadv 2,59 thus providing guarding potential onthe selector bankv contactsy of thev outgoing switch multiple. RelayZII!V closes anothery contact 2'I'If,

to4 connect; the; tip 225 of the: attendants switch.-

boa-rd jack witlrone conductor of the. trunk` ciry change subscribersvhave access tothe group of p cuit. A further contact on. relay 24.9contact 2'I 5', iis. closed to. prepare a locking circuit toxrelayv29aandicontact 2I6-1 closes a chain circuit.` This trunk 'circuitI isnowheld busy to outgoing callsv even though ringing' current may not as yethave been appliedv bythe main ofl'lce equipment. When the ringing periodcommences, the A.. Cf., current. is superi-mposedi across the trunkfrom. the main oli'ce through conductor 4'53-, contact 3I I, conductor,36'4, condenser 2891, alternating current' relay 2835, contact 21|V toconductor' 365, contact SI2, conductor IlI'4.V Relay 280 remainsvoperated but relay 283i operates and releases' in response to theringingY and silent periodsV ofy the ringing current. During the rstringing period, relay 283: closes acircuit to operate relay 290,. frombattery; two windings of relay 29.9' in series, contactsv 296i, 2:85-and 2-3'8y to ground. These windki-ngsof relay 290. are ofsuchk a naturethat Wind-y ing number 2 overpowers winding number Ii su'iiici'einl-y toallow the closure of'Y contact 293 which 'short circuits. winding numberI1 and allows relay 299 tooperate fully, closing its own lockingcircuit, from battery, number 2: wind-ing', contacts 298g 2I1=5 and 223to ground This relay 290 will remain operated during the subsequentringing periods and' until itslocking circuit. is`

opened, either at contact 2I5E or contact'.` 223. Another contact onrelay290, contact 29d" closes to light the answer lamp, over` a circuitfrom ground, contacts 292|" and 255, answer' lamp 2-:I8

' to battery; Another contactA 291, on relay 290 closes' a multipleground' t'othenumber 2 windfingr of relayA 2"!0 and' a further contact295, closes the circuit to relay 269?, from ground', contact 2`8`Ii,contact 295, number I winding of relay` 260 to battery. Relay 250i'operatesz over` this circuit and closes a part of" its own lockingcircuit and changes the source of the holding ground of' relay 210, fromcontact 283i? on relay 28W to contact 215 onrelay 219. During therstfsilent period which follows the iirstringing period", as well asduring all subsequent silent periods; relay 283' will releaseY butrelays N9', 260 and 280 will kremain operated under the control of' thevhigh resistance relay 289, which as previously stated, willl remainoperated under the control ofv ground from the main exchange over thetrunk" conductor 354i.

The branch exchange operator answers the call by inserting a plug intothe-jack J 2. Ground through contact 229? over leadi 3.61,windingnumberY Iz to battery operates relay 320, which through Ycontacts '92!v and 322' closes a multiple conneci. tion f'orthe trunkcircuit'.

Ground? at jack J12 through- Contact. 229? also furnishes a holdingground? for relay 2III, to-hold relay 2:IIl in operated position forther period of the plug remain- .ing in. thejack. The closed' loop oftheplug' circu-i-tfis now across: the tip 226 and ring 225y of the jack J 2which causesv relay 220i to operate over' theA following circuit:ground, number 2 winding of relay 229, one winding of' repeating coil2-56, contactA 231;, jackt ring` 225,. plug ring |5624, winding IN ofcord circuitirepeating coil', contact IfIZ, both windings of relayAi129- series, contact IISI., winding: |13: of repeating coil; tip I'6Iof plug, tip 22:8 of jack JZ, .contacts 2li I1 and 2'34, one winding ofrepeatingcoil, winding number I of relay 22u', to battery. Relay 2:20operates and at contact 223.re1noves holding. ground from re.- lay 29Dandv through its contact 221.y closes7 a loop across the.trufnlcconductorsf, including` the windfing of relay 249,- over thefollowing circuiti; ground, onev winding;` of relay M95, contacts 44|ual exchange.

and 42|, bank contact 421, trunk conductor 463, contact 3| conductor364, contacts 262 and 22|, one winding of repeating coil 256, relay 240,a second winding of repeating coil 256, trunk conductor 365, contact3|2, trunk conductor 464, bank contact 42S, contacts 423 and 444 and,during the silent period, through contact'45l to battery causing relays246 and 440 to operate. Contact 222 on relay 220 closes ground throughwinding of relay 256 to battery, operating relay 250. Relay 44!! cutsoii the ringing source and extends the trunk conductors 463 and 464through contacts 442 and 443 and through the windings of the back bridgerelay 416 to ground and battery respectively. Relay 246 operates and atcontact 24| completes the locking circuit of relay 266. Relay 256operates, opens contact 255 to extinguish the answer lamp 2 l S, closesground through contact 253 to the number 2 winding of relay 2|6 andthrough contact 254 of the impulsing circuit to the rotary magnet 316 ofthe adapter which will be referred to in connection with out calls, thatis calls from branch exchange subscribers. Another contact 25|disconnects the winding of the high resistance relay 280 from the trunkconductor 364, f

The trunk circuit is now ready for the branch exchange operator to speakwith the calling party in the main exchange, which may be an automaticsubscriber or an operator in a man- It should be noted that the plugmight be inserted into the jack during the ringing period but the onlyringing current which' could reach the cord circuit and the operatorsreceiver would be the amount which would be induced between the primaryand secondary windings of the repeating coil. Thus, there would be lessnoise in the operators receiver, should the plug be placed in the jackduring the ringing period, than would be the case in many previouslydesigned circuits. The operator cornpletes the connection to the wantedbranch subscriber` through the usual circuits, not shown.

After conversation is completed, both parties restore their receiversand the operator at the branch exchange removes the plug, the conditionsthen existing will depend upon which end of the trunk is released first.

We will assume that the main exchange disconnects rst, which will openthe circuit to relay 246. Relay 24|] restores and at Contact 24| opensthe locking circuit to relay 260. Relay 269 restores, and at contact 262opens one side of the trunk circuit and at contact 25| connects groundthrough relay 24! to conductor 365, I

that is, to the other conductor of the trunk, When one side of the trunkwas opened at contact 262, the switch train in the main office restoredto normal, after which a line equipment would be seized over thefollowing circuit, busying the main exchange end of the trunk; ground,contacts 26| and 22|, one winding of repeating coil, winding of relay246, another winding of repeating coil, trunk conductor 365, contact3|2, conductor 464, contact 4|6, relay 43!! to battery. Relay 436operates and places guarding potential on the selector bank multiplefrom ground through contact 43|. Relay 243 again operates over thiscircuit and at contact 24| closes a circuit to operate relay 266, frombattery, relay 260, contacts 24|, 266 and 252, conductor 364, contact3H, conductor 463, contact 4|2 to ground at the trunk switch which hasbeen seized by the operation of the line equipment. Thus, the

trunk will be guarded until the operator re- P' moves the plug to freethe trunk for Vcalls in either direction. Y

' Should the operator remove the plug prior to the trunk being releasedby the calling party in main exchange, the following would take place;relay 320 in the adapter restores, the cord circuit loop holding relay2,20 is opened allowing relay 220 to restore and at its Contact 22|opens the trunk loop of the main exchange end giving disconnectsupervision to the main exchange, this main exchange loop includes thewinding of relay 240 which restores and at its contact 24| opens part ofthe locking circuit of relay 260. As relay- 260 is being held invoperated position from contact 2`|4 on relay 210, the high resistancerelay 280 will now be bridged across trunk conductors 364 and 365 andwill operate on current, in either direction, from the main exchange.

This bridge is maintained during the time between the restoring of relay250 and the iinal release of the slow releasing relay 210. After relay210 restores ground from contact 214 is removed from the number windingof relay 260, which restores. Relay 286 is now connected to one trunkconductor and operates over the following circuit; battery, resistancecoil 241, contacts 263 and 29|, winding of relay 280, contact 25|, trunkconductor 364, contact 3| I, conductor 463 to ground in the mainexchange at the trunk switch which is still operated as this end of thetrunk has not been released. From ground at contact 28| relay 2|0 isagain energized and through its contact 2|3 connects ground to lead 259which guards the branch exchange end of the trunk until the mainexchange end is released to make the trunk free for calls in eitherdirection.

The branch exchange subscribers have access tothe trunk to the mainexchange, through the multiple banks of the usual Strowger vertical androtary type selector and connector switches. The subscriber dials theprearranged number to cause a selector to hunt and connect the callingsubscribers line to an idletrunk such as conductors 251 and 258.Conductor 259 is the usual control conductor. Subscribers may dialdirect to other branch subscribers or to authorized numbers in the mainexchange without assistance from the 4operator, although the operatormay supervise such calls. However, should a branch exchange subscriberdial an unauthorized number, the trunk in combination with the adapterwill operate to prevent the call reaching a subscriber in the mainexchange, or to other unauthorized numbers, and the main exchange endVof the trunk will be guarded until the calling subscriber abandons thecall. The dial impulses from the calling subscribers dial are repeatedover the .trunk to the main exchange through Contact 22| and arerepeated by Contact 223 through contact 254 over conductor 369 to therotary magnet 31D in the adapter. The wiper assembly of the adapter willstep over its bank contacts in response to said impulses and, should anunauthorized number be dialed, relay 3|0 will operate to disconnect thetrunk conductors 364 and 365 from leads 463 and 464 thus preventing anyfurther impulses from being received on any main oflice switch. At thesame time relay 3H), at contact 3|3 connects ground to the trunkconductor 464 which operates the line equipment in the main exchange tocause this trunk to test busy to outgoing calls from main exchangesubscribers. Due .to the action of the weighted spring 3I'l on Vrelay3|0, there is a perceptible time interval between the opening ofcontacts 3|| and 3|2 and the moment that relay 313D operates ifully, toconnect ground yto conductor :464. During thisftime interval the main.of--V -iice switch train releases, after which ythe line .and throughits contact 254, will close `a part .of

the -circuit to relay .210. After the calling subscriber abandons thecall, relay 220 will restore and at its contact .223 will complete the-circuit .to relay `210, which will operate, closing ground throughcontact 2M to energize relay Zli. While relay 2-1-3 is 'being held byitsslow releasing feature vand after relay 250.1estores, the high`resistfancerelay 280 will be bridged across .the .trunk :conductors 64and v365 and will operate from Vthe current, regardless Vof direction offilow of n .said-:current on these-conductors, from the switch .in the.mai-n exchange which has been seized by .said une equipment. y 281,`closes ground to operate relay .2 if). Relay v2li) immediately operatesand through Contact l 2 L3 places guard-ing potential on the bank'A.multiple.V Thus the period during which the bank multiple is.ung-uarded isfurther shortened and when the last slow lreleasing relayhas fully 4restored the :trunk will -be free `for calls in eitherdirection.

When -it `is necessary .to :allocate a trunk for .the exclusive yuse ofone branch exchange subscriber, for instance for through night serviceor special .day service, the said subscribers line .is connected .at'the attendants yswitchboard t0 .a .trunk by a v#patching Ycord, whichmay 4besimply a -two conductorcord with a switchboard .plug fitted at.each end. Whenthis -is done, the following conditions would prevail;relay V2I-0 will .be .held operated .over the following circuit, ground,contact .1229 .ofvjack J2, number :l winding of relay 210 to'bat- .teryand .-at contact -2I13 4will `connect ground to .lead 259 whichprevents.this particular trunk from v'being :seized by a branch .exchangeselector, this same groundis extended to lead in the adapter. lGroundthrough ycontact 244, .lamp ,2.1.1 .to :battery .lights ythe -busysignal yon -the branch exichange operators switchboard. When the vtrunkis seized at `the main exchange, ground lover conv.ductor 3'54 operatesrelay 280. Followingseizure .ringing current is superimposedintermittingly across the two conductors, .operating relay ,283, "andat`contact 284 ground` from contact 22B is :connected through contact 294to relay l23il-to sbattery, relay 230 operates ron `each impulse ofJringingcurrent. Theoperation -of relay 2.3i) .closes contact 236furnishing ground to operate relay `2H) :which-in turn, through vcontact`2.741,operates relay 2.50. Theeoperation of relay .219, opensl at.contact.21| the connection between relay 283 and `the trunk lead `3555`to allow ,the Yringing current to follow the bypass .circut. 'Theloperation of frelay 2.3.0 completes, ,at-contact 23.3 a partofrthebypass circuit tor `the ringing-current tothe .sub- `scribers line.During the ringing .period relays 2183, 210, r2li!) .and 230 `areoperated, in addition to relay 210 and one ,-path for the ringingcurrent may be traced over the following circuit: conductor 364,contacts 262 and 232, ring 225 ofv through the winding of relay 2.80.

Relay .280, .at its contact jack J2 over one lconductor yof the patchingcord one conductor of telephone line, telephone instr-ue ment, otherconductor `of telephone line, tip 2.226 -fof jack `vJ2', contacts 2H and23.5 Itotrunk conductor 3165. Another path for ringing current -is fromconductor 364 through 4.condenser 288, winding of relay V283, contact `233 to conductor .36.5.Y

Relays '2.83 and 2.30 Iwill operate during the ringi- `ingY periodandrelease during the silent period'. However, relay 269 is now being.held .by .a amultiple ground at lContact ,Z8-I through l.contact 268,winding vof relay 260 to battery. v'Ihusrelay 280 is connected in bridgeacross the tru-nk and is jbeing held vby current Yfrom `the switchboardxat the calling :end of the trunk, which may -be automatic, manual orfrom a .toll switchboard, also this direct `current may :be 'in either.direction When the called subscriber removes :his receiver to answer'the' call this places a loop across the circuit and should this occurduring the ringing lperiod, the loop wouldoperate :the back bridge`relay in .an automatic exchange or similar circuit at ya tinan- -ualswitchboard. Should the called subscribers receiver :be .removed dur-ing:the silent period, the loop would operate relay .22,0 and at lcontact72.2i .close a circuit to 1connect 'relay .24.0 across the main:exchange end of the trunk to operate the ring cut-oi relay in thecalling exchange. When relay `22,0 operates a second contact 222, is1:closed to operate relay r250, from ground, contact 222, winding ofrelay 25B to battery. The icallingfand ycalled subscribers are .nowconnected for .conversation and .the .busy .signal :indicates `to thefoperator :of .the branch exchange switchboardthat the' .trunk is busy..Af-ter conversation is geom- -pleted .both subscribers restore their,receivers vthus Vcausing the apparatus .to return to normal aspreviously described. vCalls froma line which .is connected :byapatching `cord to a :trunk fline, will be made in the same .manner as'previousls described, with the exception athat a fewer number -ofdigits will be dialed. .As the telephone line `is connected direct withthe trunk,.;the removal .of

the receiver lby .a Ybranch -exchangegsubscriber causesa switch in themain exchangeito ',be seized andthe dialed impulse .will `directlyoperatethe switches -tofcompletethe connection.

'The above completes the ,description :of :the operation vof theinvention. VWhatis considered new and is desired to have protected :byLetters 'Patent vis lspeciiically `stated :in the appended claims.

' lWhat 1isvclaimed is:

1.V In a `telephone system, fa two -way v@trunk :extending ffrom abranch y.exchange to :a ,main ,ex-

change) means xfor seizing ythe trunk .and extending kconnectionsthereover `in .either direction, ,a

relay normally connected to 'one conductor .of

`the .trunk and operatedv when 4the trunk is :Seized at the :main.exchange to busy the trunk at the branch exchange, means responsive tothe/,seizure rv.ci .the .trunk at the ,branch Vexchange :-fo'r.disconnecting said relay lfrom said `one lconductor `and for making thetrunkrbusy Aat vthe .main Aexchange lend; ,and means responsiveto the:release .of .a

:connection extended over .the trunk yfrom vthe .branch `.exchange for4connecting said ,relay .to both-conductorsfof the .tr-unkiline to holdthe .trunk .busy at y.the branch exchange .until it is cleared fat the.main exchange.

`A 2. In -a telephonesys.tem, a two waytrunk Aextending fromalcranch toamain exchange, means for extending a connection over the trunk Vin.either direction, alre'lay vnormally connected 'to one conductor of thetrunk, means responsive to the extension of a connection from the branchto the main exchange for disconnecting the relay from said oneconductor, means responsive to the initiation of the release of branchexchange for connecting said relay in bridge of the trunk, said relaycontrolled thereafter from the main exchange to maintain the branch endof the trunk busy until cleared at main exchange.

3. In a telephone system, a two-way trunk extending from one exchange toanother exchange, selector switches in said one exchange and automaticswitches in said other exchange, means for operating a selector in saidone exchange to seize said trunk and extend a connection thereover tosaid other exchange, a relay normally connected from battery to oneconductor of the other exchange section of the trunk line, means in saidtrunk responsive to said seizure of said trunk by said selector switchfor disconnecting said relay from said one conductor and connectingground to the other conductor to seize one of said switches in the otherexchange and make the other exchange end test busy, means in said trunkresponsive to the release by the subscriber in said one exchange forconnecting said relay across said trunk conductors to guard said trunkat said other exchange.

4. In a telephone system, a two-way trunk extending from a branch to amain exchange .terminating in an operators position and multiplecontactsat said branch exchange, automatic switches in the main exchange, meanscontrolled by the operator at said position for seizing the trunk andextending a connection thereover from -the branch to the main exchange,a relay normally connected from battery `to one conductor of the mainexchange section of said trunk, means in said trunk responsive to theseizure of said trunk by the operator at said position for disconnectingsaid relay from said one conductor and placing ground on-the otherconductor for seizing a switch in said main exchange and guarding thetrunk, means in said .trunk responsive to the release of the trunk bysaid operator for connecting said relay across said trunk circuit toguard said trunk at the branch exchange multiple contacts and forseizing another switchV at the main exchange to guard the main exchangeend of the trunk.

5. In a telephone system, a two-way trunk extending from a main to abranch exchange, terminating in automatic switches in the main exchange,in anoperators position and in multiple switch bank contacts at thebranch exchange, means including said equipment for seizing the trunkand extending connections thereover in either direction, a highconnected from battery to one conductor of said trunk responsive toseizure by a switch at the main exchange, means responsivev to theoperation of said relay for busying said trunk at the branch exchangemultiple switch bank contacts and for lighting the busy `lamp on theoperators switchboard, an alternating current relay in said trunkresponsive to ringing current from the main exchange superimposed acrossbot-h conductors of. said trunk for lighting the answer lamp at saidoperators position, and means in' said trunk responsive to the operationof said alternating current relay to maintain the high resistance relayconnected to one conductor of said trunk until the operator inserts aplug at the switchboard in the jack of the trunk, and means theconnection at they resistance relay normally -for preventing an'yfurther responsive to the removal of said plug for connecting the saidhigh resistance `relay across the trunk to hold trunk busy at the branchexchange until cleared at the main exchange.

6. In a telephone system,V a 4two-way trunk circuit extending fromabranch exchange to a main exchange, means for seizing said trunk andextending connections thereover from a subscriber in the branch exchangeto the main exchange, a relay normally connected from battery to oneconductor of said trunk, an adapter associated with the trunk, means insaid trunk, after seizure by a switch at the branch exchange, foroperating the stepping switch of the adapter in accordance with theimpulse'sent from the calling subscribers dial, means in said adapterresponsive to specified groups of imlpulses from the calling subscribersdial for opening both conductors of'said trunk, removing the said relayfrom one conductor and connecting ground to the main exchange end of theother conductor impulses from passing to the main exchange and forseizing a switch at the main exchange to guard the trunk until thecalling subscriber abandons the call.

'7. In a telephone system, a two way trunk extending from a branch toamain exchange having an inductive connection therein, means for seizingthe trunk from the main exchange end and means responsive to suchseizure to busy the trunk at the branch exchange' end and signal theoperator at that end, a cord circuit at times connecting the trunk to a.subscribers line and means responsive to seizure of the trunk at themain exchange, in case said cord is in place, for preventing theoperation of said signalling means and for completing a by pass aroundsaid inductive connection whereby the ringing circuit is extended tosaid cord circuit.

8. In a telephone system, a two way trunk extending from a branch to amain exchange having an inductive connection therein, means for seizingthe trunk from themain exchange end and means responsive to such seizureto busy the trunk at the branch exchange end and signal the operator atthat end, a cord circuit at times connecting the trunk to a subscribersline, means responsive to seizure of the trunk at the main exchange, incase said cord is in fplace, for preventing the operation of saidsignalling means and for completing a by pass around said inductiveconnection whereby the ringing circuit is extended to said cord circuit,and means effective when the cord circuit is not in place for preventingany operation of said by passing means.

9. In a telephone system, a two way trunk extending from a branch to amain exchange having an inductive connection therein, means for seizingthe trunk from the main exchange end and means responsive to suchseizure to busy the trunk at the branch exchange end and signal theoperator at that end, Ya cord circuit at times connecting the trunk to asubscribers line, means including a ring down relay operated from themain exchange in case of seizure of the trunk at the main exchange, incase said cord is in place, for preventing the operation of saidsignalling means and for completing a by pass around said inductiveconnection whereby the ringing circuit is extended to said cord circuit,and means controlled by said by pass means for causing continuedoperation of said ring down relay as long as ringing current is appliedto said trunk.

KARL L. BURGENER.

